1. Field of the Invention
This invention relates to a gas-turbine blade, and more particularly to a gas turbine blade having a heat-shielding coating layer formed on its surface, and a process for manufacturing the gas turbine blade.
2. Description of the Prior Art
The blades of a high temperature gas turbine are cooled to or below the temperature which the blade material can withstand. A cooling method, such as impingement or film cooling, is usually employed to cool the blades by utilizing compressed air. The blade main body is made of an alloy and often has surfaces coated with a ceramic material, since the ceramic material is superior to the metallic material in heat resistance, though inferior in thermal shock resistance and mechanical strength. The ceramic material is used as a heat-shielding coating to lower the blade temperature.
FIG. 5 shows a gas-turbine blade of known construction. The blade comprises a main body 1 made of an alloy and having a hollow interior 2 and a wall 3 having a plurality of through holes 4. Substantially the whole outer surface of the blade body 1, excluding the holes 4, is covered with a heat-shielding coating layer 5 formed from a ceramic material. Compressed air is blown into the hollow interior 2 and out through the holes 4 to cool the blade.
The holes 4 are usually made by electric discharge machining, and have to be made before the coating layer 5 is formed, since the coating is a dielectric which does not permit electric discharge machining. The holes 4 have, therefore, to be masked when the coating layer 5 is formed. The removal of the masking material to open the holes 4 thereafter, however, results in an uneven blade surface which will cause an increased aerodynamic loss.